The present study examined whether a sit-to-stand score can be related to the force-generating capacity of knee extensor muscles. Fifty-seven subjects (28 men, 63.0±7.8 yrs, and 29 women, 64.2±7.5 yrs, means±SDs) performed a 10-repeated sit-to-stand test as fast as possible, on a steel molded chair. The time taken (T_sit-stand) was measured with a manual stopwatch. The leg length (L), defined as the distance from the great trochanter of the femur to the malleolus lateralis , was measured using a tape. A power index of the test (P_sit-stand) was calculated by using the following equation: P_sit-stand=(L−0.4)×body mass×g×10/T_sit-stand. The cross-sectional area of the quadriceps femoris muscle (CSA_KE) and the maximal voluntary isometric knee extension force (F_KE) were measured using MRI and a static myometer, respectively. There was no significant correlation between T_sit-stand and each of CSA_KE and F_KE. On the other hand, the P_sit-stand was highly correlated with CSA_KE and F_KE, even after the influence of body mass and L was statistically eliminated. These results indicate that P_sit-stand, derived from three variables of body mass, leg length, and time taken for a sit-to-stand test, can be a useful index to assess the force-generating capacity of the knee extensor muscles of elderly individuals.